New types of memory have demonstrated significant potential to compete with commonly utilized types of memory. For example, non-volatile spin-transfer torque random access memory (referred to herein as “ST-RAM”) has been discussed as a “universal” memory. ST-RAM memory includes a magnetic tunnel junction (MTJ).
MTJs are written to by flowing a large enough magnitude current through the MTJ. Therefore, it is important, when reading a MTJ, that the current not be so large that it not only reads the data in the MTJ but writes into the MTJ. The writing current magnitude required by MTJ resistance switching is mainly determined by the writing pulse width. For relatively long pulse switching (>10 ns), current magnitude agrees with the theoretical equation, which is given by Equation (1):
                              I          C                =                              I                          C              ⁢                                                          ⁢              0                                ⁢                      {                          1              -                                                (                                      kT                    E                                    )                                ⁢                                  ln                  ⁡                                      (                                          τ                                              τ                        0                                                              )                                                                        }                                              (                  Equation          ⁢                                          ⁢          1                )            
Where IC is the critical switching current, which is the minimal current required for MTJ resistance switching; IC0 is the critical switching current at 0° K; E is the magnetization stability energy barrier; τ is pulse duration time; and τ0 is the inverse of the attempt frequency. As seen from this equation, the smaller the switching current that is applied, the longer the writing pulse width that is required.
Based on Eq. (1), when the working temperature increases, the required switching current for a fixed write pulse duration will decrease. This relationship is referred to as the “Thermal Stability” issue of STRAM. Considering the process variation, the thermal stability of STRAM can be represented by the shifting of the mean of required switching current.
When actual memory is designed, the current through the MTJ is designed to ensure that most of the MTJs can be switched as expected. This is generally referred to as “corner-based” design. For example, the current through the MTJ is designed to make sure that three sigma (3σ, or 99.73%) of the MTJ can be switched successfully. After considering the design corner that covers this variation, the critical write current under high temperature may be similar or even larger than the one under low temperature. For this reason, memory designers overcompensate, e.g., a transistor that is large enough to provide enough current in both high and low temperature may be required to overcome such thermal stability issues of STRAM.